Golf ball

ABSTRACT

There is provided a golf call that allows a driver shot or other similar shots prioritizing flight distance to have a reduced spin rate so that it is neither sliced or hooked and thus provides a long flight distance and that also allows an approach shot or other similar shots prioritizing controllability to have an increased spin rate to provide enhanced controllability. When the ball is hit with a number one wood it provides a back spin rate Sw#1 of no more than 2900 rpm and when it is hit with a sand wedge it provides a back spin rate S SW  of no less than 6500 rpm, and S SW /S W#1  is no less than 2.35. The golf ball&#39;s primary natural frequency (fc) in a vibration mode in a direction of torsion is 1800 Hz to 2800 Hz, and its cover has a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to 60.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to golf balls providing low spin rates and hence long flight distances for drives and providing high spin rates and enhanced in controllability for approach shots.

[0003] 2. Description of the Background Art

[0004] For golf balls, spin rate is an important factor significantly affecting flight performance and controllability. For a drive or other similar shots intended to get long flight distance, a high spin rate is accompanied by an increased side spin. The ball is thus sliced or hooked and does not provide a flight as intended by the player. It provides an upward trajectory and hence a small flight distance. Thus for shots as described above smaller spin rates are better. By contrast, for an approach shot played in the vicinity of the putting green or other similar shots intended to get high controllability, low spin rates cause a ball hit and dropped on the ground to roll over long distances, which makes it difficult to stop the ball at an intended position. Accordingly, high spin rates are better.

[0005] To allow a shot intended to get long flight distance, a golf ball providing a low spin rate has been proposed, for example in U.S. Pat. No. 5,368,304. More specifically, the document discloses a golf ball including a core having a Reihle compression of at least 0.076 and a cover having a shore D hardness of at least 65. This approach can provide a relatively soft core and a relatively hard cover to provide a golf ball with a low spin rate. However, shots intended to get long controllability also spin at a reduced rate and are thus impaired in controllability.

[0006] Furthermore to play a shot prioritizing flight distance a golf ball providing a low spin rate has been proposed for example in U.S. Publication No. 2002/0183135A1. More specifically, the document discloses a ratio (fc/fn) of a primary natural frequency (fc) in a vibration mode in a direction of torsion to a primary natural frequency (fn) in a direction of longitudinal flexure of no less than 2.22 and no more than 2.45. This approach allows a drive with a reduced spin rate. However, it uses a cover formed of a highly resilient material and also having a relatively increased thickness. As such when an approach shot is played the ball spins at a reduced rate and is thus poor controllability.

[0007] In contrast, to play a shot that prioritizes controllability a golf ball providing a high spin rate has been proposed for example in Japanese Patent Laying-Open No. 2002-263217. More specifically, the document discloses a ratio (fc/fn) of a primary natural frequency (fc) in a vibration mode in a direction of torsion to a primary natural frequency (fn) in a direction of longitudinal flexure fn of larger than 2.45 and no more than 2.65. This approach allows an approach shot with an increased spin rate. However, the ball includes a cover set larger in complex modulus than its core, and when a drive is played the ball spins at an increased rate and thus provides a small flight distance.

[0008] Furthermore to adjust a spin rate there has been proposed a large number of multi-piece golf balls having layers with their respective hardness and rigidity profiles, thicknesses, specific gravities and a large number of other similar factors in combination. However, these multi-piece golf balls which provide a reduced spin rate for a driver shot also provide a reduced spin rate for an approach shot, and the balls which provide an increased spin rate for an approach shot also provide with an increased spin rate for a drive. Spin rates adjusted for different types of shots cannot be provided.

SUMMARY OF THE INVENTION

[0009] The present invention contemplates a golf ball capable of adjusting a spin rate for a particular shot. More specifically the present invention provides a golf call that allows a driver shot or other similar shots prioritizing flight distance to have a reduced spin rate so that it is neither sliced or hooked and thus provides a long flight distance and that also allows an approach shot or other similar shots prioritizing controllability to have an increased spin rate to provide enhanced controllability.

[0010] To achieve the above object the present invention provides a golf ball providing a back spin rate S_(W#1) of no more than 2900 rpm when the ball is hit with a number one wood, and a back spin rate S_(SW) of no less than 6500 rpm when the ball is hit with a sand wedge, and also providing a S_(SW)/S_(W#1) of no less than 2.35. Preferably, S_(W#1) is no more than 2850 rpm and a back spin rate S_(SW) is no less than 6700 rpm, and S_(SW)/S_(W#1) is no less than 2.40.

[0011] Furthermore the present invention provides a golf ball including a solid core and a cover covering the solid core, the golf ball's primary natural frequency (fc) in a vibration mode in a direction of torsion being 1800 Hz to 2800 Hz, the cover having a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to 60. Preferably, the ratio (fc/fn) of the primary natural frequency (fc) in the vibration mode in the direction of torsion to a primary natural frequency in a direction of longitudinal flexure is 1.9 to 2.5, the primary natural frequency (fc) in the vibration mode in the direction of torsion is 2000 Hz to 2600 Hz, the cover has a thickness of 0.3 mm to 0.7 mm, or the cover has a Shore D hardness of 30 to 45. Furthermore, preferably the solid core is formed of more than one layer and has an outermost layer larger in Shore D hardness than the cover.

[0012] Furthermore the present invention provides a golf ball including a solid core and a cover covering the solid core, the golf ball's primary natural frequency (fc) in a vibration mode in a direction of torsion being 1800 Hz to 2800 Hz, the cover having a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to 60, the ratio (fc/fn) of the primary natural frequency (fc) in the vibration mode in the direction of torsion to a primary natural frequency (fn) in a direction of longitudinal flexure being 1.9 to 2.5, the ball deformation being 2.6 mm to 3.1 mm when the ball experiences an initial load of 98N to a final load of 1275N, the primary natural frequency (fc) in the vibration mode in the direction of torsion being 2000 Hz to 2600 Hz, the primary natural frequency (fn) in the direction of longitudinal flexure being 900 Hz to 1100 Hz, ratio fc/fn being 2.1 to 2.4, preferably.

[0013] Furthermore in the above golf ball preferably the primary natural frequency (fn) in the direction of longitudinal flexure is 800 Hz to 1200 Hz.

[0014] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the drawings:

[0016]FIG. 1A is a schematic side view of a golf ball hit with a club, and FIG. 1B is a graph of a contact force exerted when the ball is hit with the club versus time;

[0017]FIG. 2 schematically shows an apparatus employed to measure fn;

[0018]FIG. 3 schematically shows an apparatus employed to measure fc; and

[0019]FIG. 4A is a perspective view of a jig used to fix a ball and FIG. 4B is a side view thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention provides a golf ball a golf ball providing a back spin rate S_(W#1) of no more than 2900 rpm when the ball is hit with a number one wood, and a back spin rate S_(SW) of no less than 6500 rpm when the ball is hit with a sand wedge, and also providing a S_(SW)/S_(W#1) of no less than 2.35. Preferably, S_(W#1) is no more than 2850 rpm and a back spin rate S_(SW) is no less than 6700 rpm, and S_(SW)/S_(W#1) is no less than 2.40. More preferably, S_(W#1) is no more than 2800 rpm and an S_(SW) is no less than 6750 rpm, and S_(SW)/S_(W#1) is no less than 2.45. Smaller S_(W#1) allows a driver shot to provide an increased flight distance and smaller S_(SW) allows an approach shot enhanced in controllability. Furthermore, larger S_(SW)/S_(W#1) allows a larger range of adjustment of a spin rate to accommodate different types of shots.

[0021] Behaviors of a golf ball hit with a relatively large force for example for a driver shot are described with reference to FIGS. 1A and 1B. FIG. 1A is a schematic side view of a golf ball hit with a club, and FIG. 1B is a graph of a contact force (Ft) exerted when the ball is hit with the club versus time. Initially the golf ball has torsion in a surface thereof contacting the club's face and the contact force Ft is exerted in a direction to backspin the ball. This force peaks at a time point and thereafter decreases and reverses and the contact force is exerted in a direction to topspin the ball. Furthermore, this force peaks and thereafter decreases and when the force attains zero the ball launches off the club's face into the air. Note that a larger primary natural frequency in a direction of torsion (a primary natural frequency (fc) in a vibration mode in a direction of torsion allows the ball in contact with the club's face to have its direction of rotation to be reversed at an earlier timing and an impulse Rb acting for top spin increases. The ball's back spin rate is thus reduced.

[0022] Accordingly to provide a reduced back spin rate at the driver shot it is necessary to exert large force for top spin while the ball and the club's face contact each other. This is done by allowing the ball and the face to contact each other for an increased period of time and reducing a time elapsing before force reverses from the direction for back spin to that for top spin.

[0023] By contrast, when an approach shot or other similar shot is played with a relatively small force, the reversal from back spin to top spin occurs after the ball launches off the club's face. As such, the primary natural frequency (fc) in a vibration mode in a direction of torsion is irrelevant to the approach shot's back spin rate.

[0024] An approach shot's back spin rate is significantly affected by Shore D hardness of a cover of the ball located at an outermost layer of the ball. More specifically, small Shore D hardness increases friction between the ball and the club's face. The club can efficiently transfer force to the ball to backspin it and an increased back spin rate can thus be achieved. As such, to provide an approach shot with an increased back spin rate smaller Shore D hardness of the cover is better.

[0025] Accordingly the present golf ball includes a solid core and a cover covering the solid core, and the golf ball's primary natural frequency (fc) in a vibration mode in a direction of torsion is 1800 Hz to 2800 Hz and the cover has a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to 60.

[0026] For a drive, a larger primary natural frequency (fc) in a vibration mode in a direction of torsion allows a golf ball in contact with a club's face to have its direction of rotation reversed at an earlier timing so that an impulse acting for top spin increases and a reduced back spin rate is accordingly provided. If fc is smaller than 1800 Hz then before the reversal is achieved an increased period of time is required and an increased back spin rate is thus provided. If fc is larger than 2800 Hz then torsional rigidity is excessively increased, resulting in an uncomfortable feel at impact.

[0027] To allow an approach shot to have an increased back spin rate smaller Shore D hardness of the cover is better. If the cover has a reduced Shore D hardness, the ball has reduced rigidity in the direction of torsion and fc would be reduced. If the cover has a thickness of 1.3 to 2.5 mm and a Shore D hardness of 20 to 60, fc will be reduced to 1000 to 1700 Hz. If the cover has the same thickness and a Shore D hardness of 60 to 80, then fc will be 1700 to 2500 Hz, however an approach shot will have a reduced back spin rate. Accordingly, the cover is adapted to have a thickness of 0.2 to 1.3 mm and a Shore D of 20 to 60.

[0028] Furthermore the present golf ball includes a solid core and a cover covering the solid core. The ball's primary natural frequency (fc) in a vibration mode in a direction of torsion is 1800 Hz to 2800 Hz. The ball's primary natural frequency (fn) in a direction of longitudinal flexure is 800 Hz to 1200 Hz. The cover has a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to 60. Furthermore, preferably the ratio (fc/fn) of the primary natural frequency (fc) in the vibration mode in the direction of torsion to the primary natural frequency (fn) in the direction of longitudinal flexure fn is 1.9 to 2.5.

[0029] Thus, larger fc allows a golf ball in contact with a club for a drive to have its direction of rotation reversed from back spin to top spin at an earlier timing. An increased impulse acting for top spin and hence a reduced back spin rate are provided. Accordingly, fc is 1800 to 2800 Hz, preferably 2000 to 2600 Hz. If fc is smaller than 1800 Hz then before the reversal is achieved an increased period of time is required and an increased back spin rate is thus provided. If fc is larger than 2800 Hz then torsional rigidity is excessively increased, resulting in a hard and hence uncomfortable feel at impact.

[0030] Furthermore, reduced rigidity in the longitudinal direction and reduced fn allow the ball and the club to contact each other for an increased period of time. Accordingly, fn is 800 to 1200 Hz, preferably 900 to 1100 Hz. For an fn of less than 800 Hz the ball and the club contact each other for an increased period of time. A reduced back spin rate can be achieved, although at impact the player will experience a heavy and hence uncomfortable feel. For an fn of larger than 1200 Hz, the ball and the club contact each other for a reduced period of time and at impact the player will experience a hard and hence uncomfortable feel.

[0031] Furthermore if fn is reduced simply by reducing the ball's rigidity, fc is also reduced and the ball's back spin rate is not reduced. If fc is increased simply by increasing the ball's rigidity, fn is also increased and the ball's back spin rate is not reduced. To allow their balance to be an optimum, fc/fn is 1.9 to 2.5, preferably 2.1 to 2.4.

[0032] The present invention uniquely involves physical constants, the primary natural frequency (fn) in a direction of longitudinal flexure and the primary natural frequency (fc) in a vibration mode in a direction of torsion. These frequencies are measured as will now be described hereinafter. Initially, the primary natural frequency (fn) in a direction of longitudinal flexure is measured as described hereinafter with reference to FIG. 2.

[0033] (1) Grind a golf ball G to be flat, circular, and 10 mm in diameter, and fix the ground portion with instant adhesive to a vibrator 17 on an attachment 17 a at a support 17 b;

[0034] (2) Attach an acceleration pickup 19 under attachment 17 a;

[0035] (3) Operate vibrator 17 to vibrate golf ball G to measure vibration rate V of the golf ball via a reflective tape 20 by means of a laser radiation unit 14, a manipulator 12 and a laser Doppler vibrometer 11. This utilizes a principle of a known laser Doppler vibrometer. Note that the reflective tape 20 is a Scotch light trade name reflection tape of SUMITOMO 3M Limited and approximately 5 mm by 5 mm of the tape was stuck on the ball to have a reflective surface thereof facing the laser radiation;

[0036] (4) Transmit a voltage signal from acceleration pickup 19 to a power amplifier 15 which in turn amplifies the signal which is in turn taken into an FFT analyzer 13. Meanwhile, take the measured rate V from laser Doppler vibrometer 11 into FFT analyzer 13;

[0037] (5) Calculate a frequency transfer function G(s) from an acceleration A measured by FFT analyzer 13 and rate V, according to the following expression:

G(s)=Fourier transform of output rate V/Fourier transform of input acceleration A; and

[0038] (6) from frequency transfer function G(s), read as a frequency having a maximal value the highest peak value of peaks indicated for a range in frequency of 400 to 4000 Hz. Note that in FIG. 2, a vibrator amplifier 16 controls the vibration amplitude of the vibrator 17 and has a function amplifying a voltage signal output from FFT analyzer 13.

[0039] The primary natural frequency (fc) is measured in a manner, as described hereinafter with reference to FIG. 3.

[0040] (a) Arrange golf ball G on attachment 7 a via a ball fixing jig 8. Fix the ball to jig 8 at a flange 8 b, separate from the jig's base 8 a. To fix golf ball G to flange 8 b, grind the ball to be flat, circular, and 10 mm in diameter and apply instant adhesive on the ground portion and thus fix the ball to flange 8 b. Position the ball such that an extension of a laser beam R passing through the ball reaches base 8 a at a point P.

[0041] Herein, ball fixing jig 8 is formed of a material and have dimensions, as follows: Ball fixing jig 8 is formed of stainless steel (SUS) and weighs 379.5 g, and, in a perspective view, as shown FIG. 4A, and in a side view, as shown in FIG. 4B, it is formed of vertical flange 8 b and base 8 a. Each portion has dimensions, as follows:

[0042] Base

[0043] L: 93.9 mm

[0044] L1: 68.9 mm

[0045] L2: 36.37 mm

[0046] W: 25mm

[0047] H2: 15 mm

[0048] Vertical Flange

[0049] H: 47.35 mm

[0050] H1: 22.35

[0051] FL: 15 mm

[0052] (b) Attach acceleration pickup 9 under attachment 7 a;

[0053] (c) Operate vibrator 7 to vibrate golf ball G to measure vibration rate V′ of the golf ball via a reflective tape 10 by means of a laser radiation unit 4, a manipulator 2 and a laser Doppler vibrometer 1. This utilizes a principle of a known laser Doppler vibrometer. Note that the reflective tape is a Scotch light (trade name) reflection tape of SUMITOMO 3M Limited and approximately 5 mm by 5 mm of the tape was stuck on the ball to have a reflective surface thereof facing the laser radiation;

[0054] (d) Transmit a voltage signal from acceleration pickup 9 to a power amplifier 5 which in turn amplifies the signal which is in turn taken into an FFT analyzer 3. Meanwhile, take the measured rate V′ from laser Doppler vibrometer 1 into FFT analyzer 3; and

[0055] (e) Calculate a frequency transfer function G′(s) from an acceleration A′ measured by the FFT analyzer and rate V′, according to the following expression:

G′(s)=Fourier transform of output rate V′/Fourier transform of input acceleration A′.

[0056] Herein from the above frequency transfer function the highest peak value of peaks indicated for a range in frequency of 400 to 4000 Hz is read as a frequency having a maximal value. The specification of the equipment to measure frequencies fn and fc are described in Table 1. TABLE 1 Equipment used to measure fn, fc Equipment Manufacturer & type Laser Doppler DANTEC Co., Ltd. vibrometer TRACKER MAIN UNIT TYPE55 N21 Manipulator DANTEC Co., Ltd. 60X24 FFT analyzer HEWLETT PACKARD COMPANY DYNAMIC SIGNAL ANALYZER 3562A Power amp PCB PIEZOTRONICS Inc. MODEL 482A18 Vibrator amp SHINNIPPON SOKKI POWER AMPLIFIER TYPE 360-B Vibrator SHINNIPPON SOKKI 513-A Acceleration PCB PIEZOTRONICS Inc. pickup MODEL 352B22

[0057] In accordance with the present invention a golf ball preferably has a solid core formed of more than one layer, preferably 3 to 7 layers in particular, although it is not limited thereto. When the core is formed of more than one layer, a layer closer to the core's surface preferably has a complex modulus of a larger value, since fc can be increased.

[0058] Furthermore, the solid core is formed of material including rubber and/or resin, although it is not limited thereto. The rubber is obtained by hot-pressing and molding a rubber composition formed mainly of base rubber, a co-curing agent and a crosslinking initiator. The base rubber is polybutadiene with a 1,4 cis bonding of no less than 40%, preferably no less than 70%, more preferably no less than 90%, which is advantageous for restitution. Furthermore, natural rubber, styrene-butadiene rubber, isoprene rubber, ethylene-propylene rubber or the like may also be blended.

[0059] As the co-curing agent, α, β-unsaturated carboxylic acid of a carbon number of 3-8 or a metal salt thereof can be used. Metal salts of acrylic acid and methacrylic acid are preferable. The metal salt includes zinc salt, magnesium salt, calcium salt, aluminum salt or sodium salt, and zinc salt is preferable. Furthermore, 20 to 50 parts by mass of the crosslinking initiator can be used relative to 100 parts by mass of the base rubber.

[0060] The cross linking initiator is provided by organic peroxide including dicumylperoxide, 1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-(t-butylperoxy) hexane, di-t-butylperoxide, preferably dicumylperoxide. Furthermore, 0.2 to 5 parts by mass, preferably 0.3 to 3 parts by mass of the crosslinking initiator can be used relative to 100 parts by mass of the base rubber.

[0061] Furthermore the above described rubber composition can have diphenyldisulfide, dipentachlorodiphenyldisulfide or other similar organic sulfur compound added thereto to achieve enhanced restitution performance. Furthermore the above described rubber composition may also have zinc oxide, barium sulfate, clay or other similar inorganic filler, an antioxidant, a powdery colorant and the like blended together. The above described materials blended together are mixed using a roll, a kneader, a Bunbury mixer or the like to prepare a rubber composition which is in turn introduced into a spherical die and it pressurized and heated for vulcanization. The vulcanization can be effected under conditions set as appropriate. Normally, it is performed at 130 to 200° C. for 10 to 60 minutes, preferably at 150 to 170° C. for 10 to 40 minutes.

[0062] Furthermore, the solid core is formed of resin including ionomer resin, thermoplastic olefin, styrene, ester, amide or urethane elastomer, thermosetting urethane or urea resin, or the like. To increase fc and enhance impact resilience and durability, ionomer resin or thermoplastic ester, amide or urethane elastomer is preferable. Furthermore, to increase fc, short organic fiber, such as nylon fiber, acrylic fiber, polyester fiber, or aramid fiber may be blended together.

[0063] The solid core has a surface preferably of a Shore D hardness of 60 to 80, more preferably 65 to 75. If Shore D hardness is smaller than 60, fc is reduced, and if Shore D hardness is larger than 80 then the player at impact experiences a hard and hence uncomfortable feel. The solid core preferably has a diameter of 37.9 to 42.4 mm. The solid core experiencing an initial load of 98N to a final load of 1275N preferably deforms by 2.20 to 3.50 mm, more preferably 2.30 to 3.30 mm. Such deformation of less than 2.20 mm provides a hard feel at impact and that exceeding 3.50 mm provides impaired restitution performance and hence a heavy feel at impact.

[0064] In accordance with the present invention a golf ball includes a cover formed of material including ionomer resin, thermoplastic olefin, styrene, ester, amide or urethane elastomer, thermosetting urethane or urea resin, or the like, although it is not limited thereto. To mold a satisfactorily durable and thin cover, thermosetting urethane resin is preferable.

[0065] The thermosetting urethane resin contains isocyanate group ended urethane prepolymer and a curing agent of a polyamine compound or a polyol compound. The prepolymer's isocyanate group or the curing agent is not particularly limited. If necessary, oleic acid, lauryl acid or other similar aliphatic carboxylic acid, triethylamine or other similar tertiary amine, dilaurylditinlaurylate or other similar tin catalyst, titanium oxide or other similar pigment, and a barium sulfate or other similar filler, an antioxidant, an ultraviolet absorption agent, a photo-stabilizer, a fluorescent brightener, and other additive may be blended together.

[0066] The cover has a thickness of 0.2 to 1.3 mm, preferably 0.3 to 0.7 mm. If the cover has a thickness of less than 0.2 mm, it is extremely thin and difficult to mold and is also reduced in tearing strength and provides poor durability. If the cover has a thickness exceeding 1.3 mm, fc is reduced.

[0067] The cover has a Shore D hardness of 20 to 60, preferably 30 to 45. If Shore D hardness is smaller than 20, fc is small and rebound performance is also impaired. If Shore D hardness is larger than 60 then an approach shot has a reduced back spin rate and is thus poor in controllability.

[0068] The above described solid core is covered with the above described cover to form a golf ball. When this ball experiences an initial load of 98N to a final load of 1275N it deforms preferably by 2.20 to 3.50 mm, more preferably 2.30 to 3.30 mm, still more preferably 2.60 to 3.10 mm. If such deformation is less than 2.20 mm the player experiences a hard feel at impact. If such deformation exceeds 3.50 mm, rebound performance is impaired and flight distance reduced disadvantageously.

[0069] Note that the golf ball may have a surface dimpled, painted, marked or the like as required.

EXAMPLES

[0070] Hereinafter examples 1-13 and comparative examples 1-10 will be referred to to more specifically describe the present invention.

[0071] (1) Production of Solid Core

[0072] The components shown in Table 2 are blended together to provide rubber compositions (of blends A-F) or resin compositions (of blends G-J) to obtain a spherical solid core having a configuration shown in Table 3 or 4 and a diameter of 39.0 to 42.0 mm. Blends A-F are molded by being heated and compressed at 160° C. for 30 minutes and blends G-J are injection molded at 240° C.

[0073] (2) Production of Ball

[0074] The resin compositions of blends G and K-M of Table 2 are used to cover the above described solid core to obtain a golf ball having a specification of Table 3 or 4. Blend G is injection-molded-at 240° C. Blends K-M are molded as follows: initially, the materials as indicated in Table 2 to be blended are mixed and agitated to obtain a viscous, liquid, covering composition. This covering composition is rapidly introduced into a semispherical die holding the solid core. Then it is inverted and joined with another spherical die having the covering composition introduced therein. The composition is thus cured and molded.

[0075] Thereafter, the obtained golf ball is removed from the die and has fins removed and then has a surface painted white and painted transparently. The obtained golf ball has a diameter of 42.8 mm and a mass of 45.2 to 45.7 g. TABLE 2 A B C D E F G H I J K L M BR18 100 100 100 100 100 100 — — — — — — — zinc diacrylate 10 20 30 33 36 44 — — — — — — zinc oxide 28 24.4 20.9 19.8 18.7 16 — — — — — — — dicumylperoxide 0.8 0.8 0.8 0.8 0.8 0.8 — — — — — — — diphenyldisulfide 0.5 0.5 0.5 0.5 0.5 0.5 — — — — — — — Hi-milan 1605 — — — — — — 50 50 — — — — — Hi-milan 1706 — — — — — — 50 — — — — — — Hi-milan 1855 — — — — — — — 50 100 — — — — Surlyn ® 8140 — — — — — — — — — 50 — — — Surlyn ® 9120 — — — — — — — — — 50 — — — Adiprene ® LF800A — — — — — — — — — — 100 — — Adiprene ® LF900A — — — — — — — — — — — 100 — Adiprene ® LF650D — — — — — — — — — — — — 100 Lonzacure ® M-CDEA — — — — — — — — — — 12.4 16.2 31.2 oleic acid — — — — — — — — — — 0.1 0.1 0.1 titanium oxide — — — — — — 2 — 2 2 2 2 2

[0076] The materials shown in Table 2 to be blended are specified as follows:

[0077] BR18: high cis-1,4 polybutadiene (cis content: no less than 96%) available from JSR Corporation

[0078] zinc diacrylate: ZNDA-90S available from Nihonjoryu

[0079] zinc oxide: Ginrei R available from TOHO ZINC Co., LTD.

[0080] dicumylperoxide: percumyl D available from NOF CORPORATION

[0081] diphenyldisulfide: available from SUMITOMO SEIKA CHEMICALS CO., LTD.

[0082] Hi-milan 1605: Na neutralized ionomer available from DU PONT-MITSUI POLYCHEMICALS

[0083] Hi-milan 1706: Zn neutralized ionomer available from DU PONT-MITSUI POLYCHEMICALS

[0084] Hi-milan 1855: Zn neutralized ionomer available from DU PONY-MITSUI POLYCHEMICALS

[0085] Surlyn® 8140: Na neutralized ionomer available from DU PONT

[0086] Surlyn® 9120: Zn neutralized ionomer available from DU PONT

[0087] Adiprene® LF800A: TDI-PTMG prepolymer (NCO content=2.9% by mass, free TDI≦0.1% by mass) available from UNIROYAL CHEMICAL

[0088] Adiprene® LF900A: TDI-PTMG prepolymer (NCO content=3.8% by mass, free TDI≦0.1% by mass) available from UNIROYAL CHEMICAL

[0089] Adiprene® LF650D: TDI-PTMG prepolymer (NCO content=7.7% by mass, free TDI≦0.1% by mass) available from UNIROYAL CHEMICAL

[0090] Lonzacure® M-CDEA: 2,2′-dichloromethane-3,3′,5, 5′-tetramethyl-4,4′-diaminodiphenylmethane available from Lonza

[0091] oleic acid: reagent, class 1 available from Wako Pure Chemical Industries, Ltd.

[0092] titanium oxide: reagent available from Wako Pure Chemical Industries, Ltd.

[0093] (3) Test Method

[0094] (a) Deformation by compression: Deformation of a golf ball or a core compressed when it experiences an initial load of 98N to a final load of 1275N was measured in millimeters.

[0095] (b) W#1 flight distance: A number one wood club of titanium (W#1: XXIO TWIN AX-SOLE W#1 available from SUMITOMO RUBBER INDUSTRIES, LTD. (loft angle: 10°, lie angle: 56°, club length: 45 inches, shaft: carbon shaft (XXIO MP200 carbon shaft S), head's material: titanium alloy)) was attached to a swing robot and a golf ball was hit thereby at a head speed of 45 m/s to measure in meters the distance from the point at which the ball was hit and to that at which the ball stops.

[0096] (c) Back spin rate (S_(W#1) or S_(SW)): A number one wood club of titanium (W#1: XXIO TWIN AX-SOLE W#1 available from SUMITOMO RUBBER INDUSTRIES, LTD. (loft angle: 10°, lie angle: 56°, club length: 45 inches, shaft: carbon shaft (XXIO MP200 carbon shaft S), head's material: titanium alloy)) or a sand wedge (SW: SRIXON I-201 produced by SUMITOMO RUBBER INDUSTRIES, LTD. (loft angle: 57°, lie angle: 63°, club length: 35.5 inches, shaft: carbon shaft (Tour Black V23 S), head's material: soft steel)) was attached to a swing robot and a golf ball was hit thereby at a prescribed head speed (of 45 m/s for W#1 and 21 m/s for SW). The hit ball was continuously photographed and therefrom a back spin rate (rpm) was obtained.

[0097] (d) Primary natural frequency (fn) in a direction of longitudinal flexure: measured in accordance with the method described above as shown in FIG. 2.

[0098] (e) Primary natural frequency (fc) in a mode of vibration in a direction of torsion: measured in accordance with the method described above as shown in FIG. 3.

[0099] Characteristics of examples 1-13 and those of comparative examples 1-10 are summarized as shown in Tables 3 and 4, respectively. TABLE 3 Examples 1 2 3 4 5 6 7 8 9 10 11 12 13 Core 1st layer A A A A A A A A A A B A A Diameter (mm) 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 2nd layer C C D D D D D D D E D D D Thickness (mm) 3.6 3.6 3.6 3.6 3.6 3.6 2.0 3.6 3.6 3.6 3.6 3.6 3.6 3rd layer G G F G G J J G G G G G J Thickness (mm) 1.0 1.4 1.4 1.4 1.2 1.2 2.8 1.0 0.8 1.2 1.2 1.2 1.2 Deformation by 3.24 3.02 3.06 2.63 2.78 2.60 2.43 2.98 2.98 2.44 2.79 2.78 2.60 compression (mm) Core diameter 41.2 42.0 42.0 42.0 41.6 41.6 41.6 41.2 40.8 41.6 41.6 41.6 41.6 (mm) Cover L L L L L L L L L L L K K Thickness (mm) 0.8 0.4 0.4 0.4 0.6 0.6 0.6 0.8 1.0 0.6 0.6 0.6 0.6 Shore D hardness 40 40 40 40 40 40 40 40 40 40 40 33 33 f n (Hz) 831 915 921 1049 1025 1060 1132 927 945 1168 1094 1022 1053 f c (Hz) 1883 2158 1834 2360 2230 2430 2510 2068 2100 2265 2121 2120 2300 f c/f n 2.27 2.36 1.99 2.25 2.18 2.29 2.22 2.23 2.22 1.94 1.94 2.07 2.18 Deformation by 3.20 3.00 3.05 2.60 2.75 2.57 2.40 2.95 2.93 2.40 2.75 2.77 2.58 compression (mm) W#1 flight distance (m) 238 248 246 246 245 248 249 245 244 247 245 244 246 S_(w#1) (rpm) 2620 2540 2790 2680 2700 2600 2530 2710 2720 2800 2830 2730 2730 S_(sw) (rpm) 6800 6770 6730 6790 6790 6770 6760 6820 6840 6840 6820 6880 6980 S_(sw)/S_(w#1) 2.60 2.67 2.41 2.53 2.51 2.60 2.67 2.52 2.51 2.44 2.41 2.52 2.56

[0100] TABLE 4 Comparative Examples 1 2 3 4 5 6 7 8 9 10 Core 1st layer E C C A A A A A A A Diameter (mm) 39.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 32.0 2nd layer — F D C C D D D D D Thickness (mm) — 3.8 3.6 2.5 5.0 3.6 3.6 3.6 3.6 3.6 3rd layer — — G G — F J H I J Thickness (mm) — — 1.0 1.0 — 1.2 1.7 1.2 1.2 1.2 Deformation by 3.21 2.89 2.19 2.71 3.53 3.10 2.35 2.86 2.93 2.78 compression (mm) Core diameter (mm) 39.0 39.6 41.2 39.0 42.0 41.6 42.6 41.6 41.6 41.6 Cover L L M L L L L L L G Thickness (mm) 1.9 1.6 0.8 1.9 0.8 0.6 0.1 0.6 0.6 0.6 Shore D hardness 40 40 64 40 40 40 40 40 40 68 f n (Hz) 934 1054 1289 1076 759 950 1256 992 1010 1224 f c (Hz) 1581 1824 2280 1825 1343 1750 2420 1740 1660 2520 f c/f n 1.69 1.73 1.77 1.70 1.77 1.84 1.93 1.75 1.64 2.06 Deformation by 3.10 2.82 2.15 2.64 3.46 3.07 2.35 2.83 2.90 2.39 compression (mm) W#1 flight distance (m) 231 235 231 232 228 239 248 240 238 247 S_(w#1) (rpm) 3150 3030 3190 3170 3000 2940 2590 2920 3170 2620 S_(sw) (rpm) 6830 6820 5840 6830 6620 6720 5530 6800 6850 5520 S_(sw)/S_(w#1) 2.17 2.25 1.83 2.15 2.21 2.29 2.14 2.33 2.16 2.11

[0101] For examples 1-13, fc, fn, fc/fn, designing the cover's thickness, Shore D hardness and the like to fall within a predetermined range allows a golf ball with S_(W#1), S_(SW), and S_(SW)/S_(W#1) falling within the claimed range. By contrast, comparative examples 1 and 5 are associated with small fc, comparative example 9 with small fc/fn and comparative examples 1 and 4 provide an excessively thick cover, and thus all provide increased S_(W#1) and hence small flight distance. Furthermore, comparative example 7 provides an extremely thin cover, providing a low S_(SW) and poor controllability. Comparative examples 3 and 10 provide a cover with large Shore D hardness and a large fn is provided. Accordingly, S_(SW) is small and poor controllability is provided.

[0102] As described above in the present invention fc, fn, fc/fn, a cover's thickness, Shore D hardness and the like can be designed to fall within a predetermined range and S_(W#1), S_(SW), and S_(SW)/S_(W#1) prepared to fall within a predetermined range to provide a golf ball capable of adjusting a spin rate for different types of shots, i.e., a golf call that allows a drive or other similar shots prioritizing flight distance to have a reduced spin rate so that it is neither sliced or hooked and thus provides a long flight distance and that also allows an approach shot or other similar shots prioritizing controllability to have an increased spin rate to provide enhanced controllability.

[0103] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A golf ball providing a back spin rate S_(W#1) of no more than 2900 rpm when the ball is hit with a number one wood, and a back spin rate S_(SW) of no less than 6500 rpm when the ball is hit with a sand wedge, and also providing a S_(SW)/S_(W#1) of no less than 2.35.
 2. A golf ball providing a back spin rate S_(W#1) of no more than 2850 rpm when the ball is hit with a number one wood, and a back spin rate S_(SW) of no less than 6700 rpm when the ball is hit with a sand wedge, and also providing a S_(SW)/S_(W#1) of no less than 2.40.
 3. A golf ball including a solid core and a cover covering said solid core, the golf ball's primary natural frequency (fc) in a vibration mode in a direction of torsion being 1800 Hz to 2800 Hz, said cover having a thickness of 0.2 mm to 1.3 mm and a Shore D hardness of 20 to
 60. 4. The golf ball of claim 3, wherein a ratio (fc/fn) of said primary natural frequency (fc) in the vibration mode in the direction of torsion to a primary natural frequency (fn) in a direction of longitudinal flexure is 1.9 to 2.5.
 5. The golf ball of claim 3, wherein said primary natural frequency (fc) in the vibration mode in the direction of torsion is 2000 Hz to 2600 Hz.
 6. The golf ball of claim 3, wherein said cover has a thickness of 0.3 mm to 0.7 mm.
 7. The golf ball of claim 3, wherein said cover has a Shore D hardness of 30 to
 45. 8. The golf ball of claim 3, wherein said solid core is formed of more than one layer and has an outermost layer larger in Shore D hardness than said cover.
 9. The golf ball of claim 3, wherein said primary natural frequency (fn) in the direction of longitudinal flexure is 800 Hz to 1200 Hz.
 10. The golf ball of claim 3, wherein the ball deformation is 2.20 mm to 3.50 mm when the ball experiences an initial load of 98N to a final load of 1275N.
 11. The golf ball of claim 4, wherein said primary natural frequency (fc) in the vibration mode in the direction of torsion is 2000 Hz to 2600 Hz and said primary natural frequency (fn) in said direction of longitudinal flexure is 900 Hz to 1100 Hz, and the ratio (fc/fn) is 2.1 to 2.4. 